Equine genetics: an important diagnostic tool

The initial sequencing of the horse genome was complete in January 2007 and the final assembly of the genome was completed in August 2007.

Horses have approximately 20,000 different genes. Each horse has two copies of every gene; one is received from the sire and the other is received from the dam. The combination of genes that a horse has is referred to as its genotype. There are two possible states of a gene: homozygous and heterozygous. When a gene is homozygous, both copies for that gene are the same. When a gene is heterozygous, each copy is different. Observable characteristics such as coat color are a result of the horse's genotype and we refer to these characteristics as the horse's phenotype (example: bay phenotype versus palomino phenotype). Likewise, some diseases are also a result of the horse's genotype. Greater than 30 genetic disorders have been characterized in the horse to date.

Simple genetic diseases are often the result of a single mutation in a single gene. Simple genetic diseases are referred to as “Mendelian” diseases. Popularity of a particular stallion leads to a predominance of certain sire lines. An average equine stallion might produce 250 offspring over their lifetime with a possible maximum of 3,000 or more. If highly popular sires carry a genetic mutation, their descendents can very rapidly produce many thousands of related offspring carrying the same mutation. Examples of single gene, simple Mendelian genetic diseases include:

Hyperkalemic Periodic Paralysis

Severe Combined Immunodeficiency

Overo Lethal White Syndrome

Junctional Epidermolysis Bulosa

Glycogen Branching Enzyme Deficiency

Malignant Hyperthermia

Hereditary Equine Regional Dermal Asthenia

Type 1 Polysaccharide Storage Myopathy

Mendelian diseases can occur in either a dominant or recessive fashion. Dominant traits occur when at least one copy of the gene contains the disease causing mutation. Recessive traits require both gene copies to contain the disease causing mutation.

Complex genetic diseases, on the other hand, can occur as a result of mutations in more than one gene and may require additional non-genetic environmental factors to be present in order to cause expression of the disease. Examples of complex genetic diseases include:

Recurrent Airway Obstruction “heaves”

Equine Metabolic Syndrome

Osteochondrosis Dessicans Cervical Stenotic Myelopathy “wobblers”

Although the list of currently known genetic mutations in horses is small, the list is likely to grow at a rapid rate in the next decade, thanks to equine genome sequencing and mapping efforts. The initial sequencing of the horse genome was complete in January 2007 and the final assembly of the genome was completed in August 2007.

The availability of the horse genome sequence will allow for the creation of tools that will allow equine genetic researchers to rapidly identify the underlying genetic defects of many Mendelian diseases in horses. In addition to identifying simple dominantly or recessively inherited Mendelian diseases, these tools will also allow researchers to study the genetic basis of complex diseases in horses.